Swimbladder properties of Cyclothone spp. in the northeast Atlantic Ocean and the Western Mediterranean Sea

Non-migratory bristlemouth fishes ( Cyclothone spp.) are the most abundant vertebrates on Earth and play an important role in biological carbon pump by remineralizing organic deep ecosystems. Acoustic data net sampling often used combination to estimate fish zooplankton biomass, but this procedure may be subject several sources of error when applied mesopelagic species. For instance, allocation echoes species has been biased not considering spp. due use nets targeting larger fish. Furthermore, acoustic properties target organisms must well understood convert density into numerical density. The characteristics a fish’s swimbladder relevant features necessary assess its properties. This study provides information six inhabiting meso- bathypelagic layers North Atlantic Ocean Mediterranean Sea, including location within body, fat tissue content, morphology, morphometry (only available for C. braueri pseudopallida ), body-mass , pallida pygmaea ). studied showed functional physoclistous swimbladder, with well-developed gas glands rete mirabile numerous capillaries case shallower (mainly distributed from 400 600 m depth), fat-invested deeper vertical distribution livida microdon content ) increased depth latitude, reducing space that could contain gas. Changes size volume during growth were analyzed species, where equivalent radius followed negative allometric relation body length. Finally, values (ρ) required neutral buoyancy (V G estimated collected between 350 550 (ρ = 1.052–1.072 g·cm −3 V 2%–4%; ρ 1.052–1.062 3.6%), sampled 450–700 layer 2.6–3.1%; 2.8%–3.25%). Results highlight change scattering behavior gas-bearing (those their swimbladder) upper zone fluid-like (with fat-filled swimbladders) northern individuals. presented manuscript parametrizing backscattering models built echo although further work is needed, particularly individuals partially invested swimbladders irregular fat-free shape.